Morgoth did influence ME east of Beleriand. The Misty Mountains were wrought by him to hinder or bar the eastward migration of the elves. I dont think Beleriand in itself was "better" than the rest of ME. What was special about it was the fact that it was the stage in which the drama of the Silm was played out.
IMO, the geological effect to Beleriand was a random effect of the aftermath of the Valar coming in and doing some business with Morgoth. One could argue possibly that the islands were meant to remain. I think it was the remoteness of Ossiriand to Angband that spared at least some of it.
my 2 cents for what its worth...